Light Heterogeneity and Plants: from Ecophysiology to Species Coexistence and Biodiversity
Light is produced by changes in the energy level of electrons (when an electron changes from a high-energy, Or excited, State to a low-energy state, Its atom will emit a photon), but its dual nature makes it scientifically puzzling: it moves through space
- PDF / 4,389,117 Bytes
- 33 Pages / 439 x 666 pts Page_size
- 32 Downloads / 185 Views
Light Heterogeneity and Plants: from Ecophysiology to Species Coexistence and Biodiversity Fernando Valladares
1 Introduction Light is produced by changes in the energy level of electrons (when an electron changes from a high -energy, or excited, state to a low-energy state, its atom will emit a photon), but its dual nature makes it scientifically puzzling: it moves through space as a wave, but when it encounters matter it behaves like a particle, the quantum (Achenbach 2001). In practice "light" is used for the portion of the electromagnetic spectrum in the vicinity of visible light. In plant biology and ecology, photosynthetically active radiation (PAR), which essentially coincides with visible light, is probably the most relevant measure of light. The PAR region is where energy is most abundant (it represents on average 43% of the solar irradiance), and it is strong enough to drive electron transport in photosynthesis, yet weak enough to avoid excessive damage to biological molecules. However, there exists no worldwide network for PAR measurements like the network of actinometric stations, where global, direct, diffuse and reflected solar radiation are measured using unified instruments and methodology and are metrologically based on the world radiometric reference (Ross and Sulev 2000). The dual nature of light, i.e., particle versus wave, affects the way light in general and PAR in particular is measured. One group of sensors measures energy (e.g., W m- 2) , the other group quanta (e.g., prnol m- 2 S-I). General conversion factors are 1.814, 1.758, 2.127, and 0.4621lmol PAR W-I for global, direct, diffuse, and reflected radiation, respectively (Ross and Sulev 2000), but these factors should be used with care since they may be different under different environmental conditions. Light provides the energy used in photosynthesis and the signals used in photoregulation of plant growth and development, and is, among the factors affecting plants, perhaps the most spatially and temporally heterogeneous (Pearcy 1999). PAR in the understory ranges from 50-80% of full sunlight under leafless deciduous trees, to 10-15% in even-aged pine stands, 2.5% in closed spruce canopies, 0.2-0.4% in dense beech forests, and even less than 0.1% in certain tropical rainforests (Barnes et al. 1998). In addition to this quantitative variability, light is also qualita-
Progress in Botany, Vol. 64 © Springer-Verlag Berlin Heid elberg 2003
440
Ecology
tively heterogeneous. A total of five basic light environments can be found in terrestrial ecosystems according to the color of light: (1) forest shade (greenish or yellow-green light due to selective absorption of red and blue by vegetation), (2) woodland shade (bluish or bluish-grey light due to the dominance of the radiation from the sky), (3) small gaps (yellowish -reddish light due to direct sunlight), (4) large gaps, open, or any habitat under cloudy conditions (whitish light due to combination of sun and sky light, or because of the dominance of the white light radiating from clou
Data Loading...
